1. Field of the Invention
The present invention relates to a readily fibrillatable fiber comprising a vinyl alcohol based polymer (abbreviated to "PVA" hereinafter) and a cellulose polymer; more specifically, the present invention relates to a fiber and a fibril, characterized in that the fiber is readily modified into a superfine fibril through the single action of chemically swelling force or mechanical stress or the combination thereof and is therefore preferable for use in wet laid or dry laid nonwoven fabrics, separators in alkaline batteries, reinforcing fibers for friction materials and reinforcing fibers for cement products.
2. Description of the Prior Art
Nonwoven fabrics comprising PVA fibers have been used conventionally as the separators of alkaline manganese batteries due to their strong alkaline resistance. Following the development of electronics and information and communication systems in recent years, far advanced performance has been demanded toward batteries, while mercury-free batteries have also been needed from the respect of pollution-free battery production and disposal. Additionally, more outstanding separating potency has been required for separators for use in batteries because of the demand for higher performance without mercury. Therefore, PVA fibers of a finer denier have been prepared for use in the separators of alkaline manganese batteries, and a PVA fiber of 0.3 denier is now commercially available. The absorptivity of alkaline solutions (namely, absorption in weight of aqueous KOH solution) as a very significant property for the separators in alkaline manganese batteries cannot sufficiently be satisfied by simply preparing a PVA fiber of a finer denier.
In order to overcome these problems, use has been made of a separator comprising a mixture of a PVA fiber of a finer denier and a polynosic fiber as one cellulose fiber with great absorptivity of alkaline solutions which is readily fibrillatable into a superfine fibril through beating. Disadvantageously, however, the polynosic fiber may cause public hazards in the production process. Additionally, the polynosic fiber has such poor beatability that the central part of the fiber remains as a thick stem in the resulting fibril. Thus, it is very difficult to recover a fibril sufficiently finely disintegrated to such an extent that the stem is also disintegrated. Hence, it has been desired a PVA fiber fibrillatable into a superfine fibril and having greater absorptivity of alkaline solutions and higher alkaline resistance.
As the reinforcing fibers of a variety of friction materials for use in automobile brakes and clutch plates, conventionally, asbestos has been used commonly in terms of the trapping performance of inorganic particles, thermal resistance, heat fusion resistance, reinforcing properties and the like. However, the use of asbestos has been put under strict regulations because of concern that asbestos may be harmful for human health. In recent years, therefore, the fibril of costly aramide fiber has been replacing asbestos. However, aramide fiber is so costly that it is only used in a limited fashion. Thus, low-cost materials with insufficient reinforcing performance, such as natural pulp, are used practically. Accordingly, a fiber has been desired which is less expensive than aramide fiber and fibrillatable so that the fiber might procure particle trapping performance, thermal resistance, heat fusion resistance and reinforcing properties in combination.
Asbestos has been used conventionally as a reinforcing fiber for cement products such as slate plate, but the use thereof is strictly regulated by the same reason as described above. PVA fibers have been used as an alternative to asbestos because the fibers have greater resistance to the alkalis in cement, but because PVA fibers have larger fiber sizes than that of asbestos, the green strength of the slate reinforced with the fibers is low. In order to supplement the strength, the fibers should be used in combination with fibrils of natural pulp and the like. If any fibrillatable PVA fiber is present, conventional laborious works required to use PVA fibers and natural pulp in combination can be eliminated.
In order to produce a superfine synthetic fiber, furthermore, a great number of attempts have been made conventionally to utilize the phase separation phenomenon of blend polymers. For example, Japanese Patent Publication No. 10617/1974, Japanese Patent Publication No. 17609/1976, Japanese Patent Application Kokai (Laid-open) No. 56925/1973 and Japanese Patent Application Kokai (Laid-open) No. 6203/1974 describe individually that a sea-islands fiber comprising a acrylonitrile polymer as the sea component and a PVA graft copolymer with acrylonitrile or a methyl methacrylate polymer as the islands component is fibrillatable through beating. But these techniques belong to modification technology of so-called polyacrylonitrile fiber comprising polyacrylonitrile as the sea component. Because polyacrylonitrile fiber is poor in terms of alkali resistance and good absorptivity of alkaline solutions, the fiber cannot be used in the utilities demanding excellent performance in these terms or the utilities demanding thermal resistance.
Japanese Patent Publication No. 31376/1972 also discloses a readily fibrillatable PVA fiber comprising a completely saponified PVA as the sea component and a partially saponified PVA as the islands component, but the fiber has a drawback such that the partially saponified water-soluble PVA is solubilized during the beating process in water for fibrillation, involving severe foaming during beating.